Non-Patent Document 1 suggests a logarithmic sensor that operates a photodiode in an open circuit as in a solar cell, and measures an output voltage. According to this suggestion, the relationship in which a potential difference caused when a current flows in the forward direction of a PN junction, or a voltage, is proportional to the logarithm of the current is utilized. This forward current is replaced with a photocurrent generated through photoelectric conversion, and the forward voltage of the PN junction is monitored. As a result, a signal generated as a result of logarithmic compression of the photocurrent can be obtained.
Patent Documents 1 through 4 suggest techniques using such logarithmic sensors. Patent Document 1 and Patent Document 2 each suggest a combination of a logarithmic sensor in a solar cell mode and a general storage-type CMOS image sensor in a space-sharing manner. Patent Document 3 and Patent Document 4 each suggest a combination of both sensors in a time-sharing manner.